I'm sorry to hear about your experiences. Just to comment, I have never once slapped any part of a student's body. It also amazes me how teachers think it's okay to do other things during the lesson, or waste the time. That is something I've experienced from some myself - I once had a teacher who talked forever and wasted so much time. I'm very conscious of this, and although with young students I'll sometimes go off on educational anecdotes, I'm always conscious, 'okay we have to get back to work now'.
I've worked with a ton of adult students. I'm very honest with them. I can show up at show them good technique and correct mistakes and talk about the pieces and teach theory, but the biggest enemy is time. Adults have jobs, kids, lives, and it takes true dedication to make the time necessary to practice. And if there's no practicing, you're wasting your money on me.
Going through a higher-end college system, I never had teachers who didn't know their stuff, but I did have some who brought me to near nervous breakdown. When I went back to college, initially it was similar - my confidence was destroyed and I was made to feel like I couldn't play a single note right. Then, I finally got someone good, and working with him my playing went to the next level.
What did he do? Well, he was surprisingly laid back. He would NEVER tell me an interpretation was 'wrong'. He would listen, and simply comment about what he found convincing, and what he didn't. For technique, I came in asking about fingering, and he showed me the real thing to think about was the movement of my arm, and how something I found undoable was actually possible with several different fingerings if my arm movement was right. He would say things like, "It's hard because you think it's hard," which sounds cheesy, but he was right - I was going into difficult passages tense, and thinking 'wow I really have to try here' when in fact I just had to loosen up more.
Not only was I able to play things I couldn't before, but if I had continued to use my old technique, I would have destroyed my hands years ago. The minimal-effort approach that he taught allows you to keep playing well into older age. In fact his teacher was around 80 years old, and still able to play just about anything. I often let my students listen to Art Tatum playing, imagining what his hands might be doing, and then show them video of what he actually does - almost nothing.
So, that's what a good teacher can bring to the table. I'm sorry you've had the experiences you've had.